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One of my favorite mental health tools is free, abundant, and accessible.

It’s sunshine.

If you’re rolling your eyes right now, I understand. A few years ago, if someone had told me sunshine would help me pull myself out of a depressive slump, I’d have laughed.

In fact, when my doctor “prescribed” me to get out into nature more often, I felt like they were dismissing the seriousness of my mental illnesses. Therapy and medicine are supposed to be the transformative treatments when depression gets really bad, right?

Not exactly. While sunshine sounds like a pseudoscientific, crunchy Band-Aid for mental illness, there’s actually a fair amount of scientific evidence that it’s important for our moods.

You probably already know the sun is a great source of vitamin D.

Vitamin D is essential for the healthy functioning of your body, and a lack of vitamin D is linked to depression — although, as this review points out, it’s not clear if a vitamin D deficiency causes depression or the other way around.

But there’s another way the sun affects our brains: our circadian rhythm.

We all have an “internal clock” called circadian rhythm that determines when we feel sleepy and when we feel alert. Humans evolved to have this 24-hour cycle in sync with the light that reaches Earth. We feel awake and alert when the sun rises. When it’s dark, our bodies release melatonin, a hormone that helps us sleep.

When it’s working properly, your circadian rhythm isn’t just about maintaining shut-eye sessions: It also helps keep your cardiovascular system, immune system, digestive system, and other bodily systems in check. (However, if you have a circadian rhythm sleep disorder, you might feel fatigued during the day and awake at night.)

“On a cellular level our bodies are functioning as they’re meant to: Our bodies interpret light as a sign of when to be awake, and it regulates our energy levels, metabolism function, our appetites, immune function, and even balances our hormones,” says Michael Hamblin, PhD.

Hamblin, an associate professor of dermatology at Harvard Medical School and a principal investigator at the Wellman Center for Photomedicine at Massachusetts General Hospital, has published more than 300 peer-reviewed articles on light therapy.

It’s no surprise that when your circadian rhythm goes out of sync, so might your mental health.

“Depression and sleep are well-known to affect one another,” says Sujay Kansagra, MD, director of Duke University’s Pediatric Neurology Sleep Medicine Program. “Insomnia is a common complaint amongst those experiencing depression. Similarly, having an underlying sleep disorder or chronic sleep deprivation can make you more prone to mood dysfunction.”

A 2013 literature review noted that “nearly all people suffering from mood disorders have significant disruptions in circadian rhythms and the sleep/wake cycle.”

More research is being conducted on exactly why the circadian rhythm can affect mental health. A recent study noted that the part of the brain involved in regulating the circadian rhythm is close to the parts that regulate mood.

Since so many of us work indoors all day and look at stimulating blue screens at night, it’s no wonder many people have circadian rhythm disorders.

Unfortunately, few of us make the connection between the circadian rhythm and mental health. But there’s a pretty easy way to get our circadian rhythm back on track.

Both Kansagra and Hamblin recommend getting a lot of sun in the morning. Simply opening your curtains wide and spending a few minutes in the sun can help.

Every morning, I have my coffee or tea while sitting outside. This makes my brain “wake up” a lot faster than if I just guzzle my coffee in my bedroom. If it’s raining or cold, I sit near a bright window.

I also moved my desk — which is where I spend most of the day — into a sunny spot so I can get enough light.

Just adding sunshine to my routine has made me feel more alert during the day and sleepier at night. It also immediately lifts my mood, which falls in line with the theory that sunshine may stimulate the production of serotonin.

Serotonin is a mood-boosting hormone, and a lack of it is associated with mood disorders. This is one possible reason many people get depressed in winter or even experience major depressive disorder with a seasonal pattern (formerly known as SAD, aka seasonal affective disorder).

Depression makes doing basic activities such as showering, eating, and going to the bathroom more difficult. And yes, I’ve been in such awful depressive slumps that standing in the sun for 5 minutes seemed impossible.

But I find that, if and when I can muster up the energy to get some sunshine, I see a huge improvement in my mood and energy levels. On my worst days, sunshine keeps me going.

On the flip side, it’s important to avoid getting too much light at night. Light signals to your brain that it should be awake, and this can affect your circadian rhythm. Kansagra recommends avoiding blue light, like from your TV or phone, for at least half an hour before bed.

Scientists have harnessed other ways to use light to correct the circadian rhythm. Hamblin is on the Scientific Advisory Board of Joovv, a company that makes devices for red light therapy, also known as photobiomodulation, which can help reset the circadian rhythm.

A red light therapy device delivers wavelengths of light to your skin. “Your body will naturally produce melatonin from red light, and medical research from many clinical trials has shown improved sleep disorders from red light therapy,” Hamblin says.

This can be helpful for people who can’t get enough sunlight during the day because of their environment (either work or natural). The device is typically used at the end of the day, ideally just before bedtime.

I’ve followed a lot of the usual sleep hygiene advice, like doing relaxing activities before bed and trying to wake up and go to sleep at the same time. While those things help, sunshine is the easiest and most effective tool in my insomnia-fighting arsenal.

But for me, and for many others, sunshine is one of the few excellent mental health tools that are free, easy to access, and effective. It’s also pretty safe, as long as you remember to wear sunscreen or sunblock.

Although it’s annoying when people give you unsolicited mental health advice along the lines of “just go outside,” there is some merit to the method.

You don’t need to hike for half a day or fly to a tropical region to enjoy the benefits of sunshine, and you don’t have to spend too long doing it. Getting 15 minutes of sunshine a day can be enough.

Make an appointment if…

If you’re struggling to fall asleep at night and you don’t feel rested in the morning, you may have a sleep disorder. It may be worth talking to your doctor to see if any underlying conditions might be affecting your health.

Sian Ferguson is a freelance writer and journalist based in Grahamstown, South Africa. Her writing covers issues relating to social justice and health. Find her on Twitter.